1. Field of the Invention
The present invention relates to signal processing, and, in particular, to .pi./4-differentially encoded quadrature phase-shifted keying (.pi./4-DQPSK) modulation.
2. Description of the Related Art
FIG. 1 shows a block diagram of a conventional digital system 100 for encoding and modulating speech-related audio signals for transmission using .pi./4-DQPSK modulation. As shown in FIG. 1, analog-to-digital converter 102 digitizes the input signal, speech encoder 104 encodes the digitized stream, channel encoder 106 adds redundant bits to the encoded data, interleaver 108 interleaves the encoded data (to enable a receiver to perform error correction in the presence of burst errors in the transmitted signals), and modulator 110 applies .pi./4-DQPSK modulation to dibits (i.e., pairs of bits) in the interleaved data stream to generate multi-bit digital values DI and DQ corresponding to the in-phase (or real) and quadrature-phase (or imaginary) parts of the modulated signal. These two digital streams DI and DQ are converted to analog signals by digital-to-analog (D/A) converters 112, and the analog signals are then filtered by low-pass filters 114 to yield the two analog output signals AI and AQ corresponding to the in-phase and quadrature-phase parts of the modulated signal.
In .pi./4-DQPSK modulation, an input stream of dibits, represented by (y,x), is converted into a modulated output stream of complex values I+jQ, where I is the in-phase or real part and Q is the quadrature-phase or imaginary part of each complex value. In typical digital implementations, this conversion process involves the application of a computationally intense finite impulse response (FIR) filter to an over-sampled data stream to generate a stream of over-sampled interpolated complex values. The degree of over-sampling in the conversion process is often limited by the finite processing capabilities of the processor used to implement the modulation scheme. As such, the level of distortion in the modulated signals may be higher than desirable. Less distortion can be achieved by using more costly processors that have greater processing capabilities to handle higher levels of over-sampling.